init: do not create dev.mnt. properties for emulated mounts

dev.mnt. properties are primarily intended for tuning parameters for
mounts such as /system and /data but don't have much use for emulated
mounts.

There are additional emulated mounts created for each user on a
device, so if too many users are created, init would otherwise create
too many dev.mnt. properties, filling the property file that backs
these properties, and preventing more properties from being
generated.

Therefore, this change stops init from creating dev.mnt. properties
for emulated mounts.

Bug: 156721033
Bug: 179111945
Test: user creation stress test doesn't create large numbers of
properties

Change-Id: I6475956719b7c938b8289189abfef661140d526d
(cherry picked from commit 7e4ce2b5)